1. Field of the Invention
The present invention relates to an electrostatic actuator that uses an electret as movable member, a shutter device that has the electrostatic actuator, an imaging module that has the shutter device, and a camera that has the shutter device.
2. Description of the Related Art
In recent years, digital cameras having a solid-state imaging element that converts an optical image of an object into electronic data have come into widespread use in various apparatuses such as mobile telephones, portable terminal apparatuses, watches, peripheral terminals of data-processing apparatuses and digital household appliance. In this trend, it is demanded that the digital camera should perform various functions.
First, it is demanded that the camera body should be thin and light.
The following two techniques are known, which make the camera body thin.
One of the techniques is to protrude the glass cover of the imaging unit incorporating the imaging element, into the shutter unit, thereby to render the camera body thin (see Jpn. Pat. Appln. KOKAI Publication No. 11-218838). The other technique is to combine the imaging element and the luminous energy adjusting plate, thus making the imaging unit thinner (see Jpn. Pat. Appln. KOKAI Publication No. 9-129859).
Secondly, it is demanded that the stroboscopic tube should be controlled with high precision.
When the camera performs strobe photographing, the stroboscopic tube light in synchronism with the full opening of the shutter. The stroboscopic tube should therefore be controlled to emit light at the very time when the shutter operates at high speed.
The following two techniques are known, which control the light-emission timing of the stroboscopic tube with high precision.
One of these techniques is to use a control means that calculates the time when the stroboscopic tube should emit light, from the time when the means for monitoring the motion of the shutter blade detects that the shutter blade starts moving (see Jpn. Pat. Appln. KOKAI Publication No. 11-15052).
The other technique is to count the drive pulses supplied to the stepping motor that drive the shutter blade, and to make the stroboscopic tube to emit light in accordance with the resultant count value (see Jpn. Pat. Appln. KOKAI Publication No. 5-36430). Thirdly, it is demanded that the shutter mechanism should be light and operate at high speed.
A technique known as meeting this demand is to use an electrostatic actuator that has an electret employed as movable member. The movable member of the electrostatic actuator can be made lighter and can thus be moved at higher speed (see Jpn. Pat. Appln. Publication No. 4-112683).